In the manufacture of semiconductor wafers, microlithography is used to pattern various layers on a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and a template such as a mask or reticle. During the exposure process a form of radiant energy, such as ultraviolet light, is directed through the template to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, thereby forming a resist mask on the wafer. The resist mask can then be used to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes.
Manufacturers in the field of integrated circuits (ICs) have been trying to reduce the geometric size of the devices (e.g., transistors or polygates) present on integrated circuits. The benefits achieved by reducing device dimensions include higher performance and smaller packaging sizes. Improving lithographic techniques provides improved resolution and results in a potential reduction of the minimum dimension. However, at small geometries, diffraction effects such as proximity effects, poor subject contrast, and poor resolution result, producing wafers with incomplete or erroneous circuit patterns.
An individual reticle can cost up to $20,000 and typically requires up to two weeks to manufacture. Mask production likewise involves substantial time and expense. The complete circuit patterning for a modern IC will typically require 10 to 20 or more reticles, and the ability to modify or repair a reticle or mask quickly saves turnaround time and cost.
The most common template defects are pattern distortions of two types: opaque defects and clear defects. Reticles and masks typically consist of an opaque thin film of a metal, such as chromium, deposited in a pattern on a transparent substrate of quartz or glass. Opaque defects, which may occur as spots, pattern extensions, bridges between adjacent patterns, or the like, are the result of opaque material such as chromium or molybdenum silicide being present in a non-pattern area. Clear defects, which generally occur as pinholes, missing parts, or breaks in the pattern, are the result of missing or inadequate layers of opaque material in a pattern area on the template.
Focused ion beams (FIBs) have been used for repair of optical masks and reticles since the mid-1980s. The ability of the FIB to accurately remove unwanted portions of the metal film and to deposit material to "edit" the pattern makes it potentially an almost ideal repair tool. A FIB exposes a template to a beam of positively charged ions, typically gallium ions, via an optic system. When a template is exposed to the ion beam, secondary ions are produced, and may be detected by the FIB machine and monitored to determine the progress of repair work. If a chromium pattern is exposed, secondary chromium ions are generated, and if a silicon or glass pattern is exposed, secondary silicon ions are generated.
Sputtering with a scanning FIB is the preferred method of opaque defect repair at small geometries, but FIB sputtering has several disadvantages. First, difficulty in precisely determining the endpoint when etching may lead to overetching and subsequent recess formation in the template substrate. Second, the high energy (25 to 50 KeV) FIB beams used can cause significant template damage during repair due to the beam's high sputter rate. In addition, significant amounts of ions from the ion beam are implanted into the template substrate during imaging and opaque defect repair, resulting in an effect called "ion staining" or "gallium staining", when a gallium ion beam is used. This effect causes local reductions of the substrate's transparency which may print on the semiconductor wafer, and/or may be identified erroneously as defects by industry-standard mask inspection equipment.
The clear defects on a photomask have been traditionally repaired on a focused ion beam (FIB) tool by carbon deposition. However, a carbon halo was always found around the actual repair. This problem is more severe when the clear repair is done on a dense geometry semiconductor, e.g., a semiconductor structure designed using a 0.18 micron or less design rule. In a worst case scenario, the carbon halo can bridge the adjacent features causing the semiconductor formed using the mask to short out. In current production flows, the carbon halo present after clear repair is typically zapped on a DRS II laser repair tool. However, because of the limit of the optical resolution, the defect is very difficult to be reworked on DRS II when the feature size is less than 0.7 or 0.8 micron. No processes have been developed to remove the carbon halo on a FIB tool. As a result, many masks have to be rejected because of unrepairable carbon halo defects.
There is needed, therefore, a method of repairing clear defects on a template that reduces or prevents overetching, that reduces the carbon halo present after clear defect repair and that reduces damage caused by ion staining.